I kind of want to sell them on BGG or eBay too
According to "Hans im Glück" that's exactly what they DON'T want to. The reason is not to be mean, but because a lot of people create poor-quality prints of self-made expansions and sell them on eBay. That alone wouldn't be a problem, but unfortunately some of the people who buy these self-made expansions think that they are official products, and complain to HiG about the bad quality of "their" products. That's why they don't want to enforce this even more by giving away huge amounts of blank tiles.
By the way, I've received my blank tiles from them, and honestly, to me they are not the best way to make your own tiles. The problem is actually the blank side, as it is completely white. And since this white side isn't just on top of the tiles, but a little bit down to their sides as well (resulting from cutting... just like the regular tiles), it makes them immediately recognizable in the stack. Play with the forest expansion? Don't want to pull a forest tile? Just avoid the ones with the white edges...
I think getting some base-sets for a few bucks on eBay might work better, as their tiles look the same as the others when in the stack.